1. Field of the Invention
The present invention relates to the sizing of pulp for use in producing liquid packaging board and more particularly it relates to the use of alkyl ketone dimer, cationic rosin size and insolubilizing agent at neutral or alkaline pH in producing liquid packaging board having outstanding resistance to edge penetration by hot hydrogen peroxide.
2. Description of the Prior Art
In recent years there has been a trend towards packaging liquid products, and in particular liquid dairy products such as milk and cream, in containers made out of coated paper-based board. The coating may be on one side of the board, but is generally on both sides. The coating is generally of polyethylene, although other substances are used.
In order to be able to function effectively in such a container, the board must be resistant to the effects of the liquid. For liquid dairy products, the most aggressive component of the liquid is generally lactic acid. The most vulnerable area of the board tends to be its cut edge.
Board manufacturers have therefore investigated ways to improve the resistance of board to edge penetration by lactic acid-containing liquids. It is known that board sized with a ketone dimer (KD) has good resistance to edge penetration by lactic acid-containing liquids.
More recently, there has been a trend towards aseptic packaging of consumable liquids and in particular liquid dairy products. In order to carry out aseptic packaging, it is necessary to sterilize the package as well as its contents. It has been proposed that containers made out of board should be sterilized by use of hydrogen peroxide solutions at elevated temperatures. It has been found that board sized with a KD has low resistance to edge penetration by hot hydrogen peroxide containing solutions. There is therefore a need for a board which has good resistance to edge penetration by such hot solutions of hydrogen peroxide, and particularly by liquids containing lactic acid following contact with hot hydrogen peroxide.
It has been shown that board sized with a cationic rosin size (CRS) or with a conventional anionic rosin size (emulsion, paste or soap) has good resistance to edge penetration by hot hydrogen peroxide containing liquids, but low resistance to edge penetration by lactic acid containing liquids.
It is well known in the art that sizing with KD's must be carried out at neutral or slightly alkaline pH's (between 7 and 8.5) in order to achieve effective sizing. Moreover, sizing with KD's can be adversely affected by the presence of some cations, such as aluminum cations.
It is also well known in the art that, in order to be effective, sizing with CRS must be carried out at acidic pH's between 4 and 6, and sizing with conventional rosin sizes must be carried out at acidic pH's between 4 and 5. Further, both sizes require the presence of an insolubilizing agent (i.e. a compound which forms an insoluble complex or salt with the size). The most commonly used insolubilizing agent is alum (A12(S04)3).
U.S. Pat. No. 4,522,686 discloses a sizing composition in the form of an aqueous dispersion containing a hydrophobic cellulose reactive sizing agent, such as a KD, fortified rosin and a water-soluble, nitrogen containing cationic dispersing agent, the last two components forming the elements of a CRS. As shown in the Examples of this patent, the dispersion is made by producing a KD emulsion using a cationic resin. A cationic emulsion of fortified rosin is then also prepared. The final dispersion is made by mixing the two emulsions in various ratios. The final dispersion is then used, with or without the addition of alum, to produce sized paper from a pulp at a pH of about 6.5. The patent shows that using such a cationic dispersion results in improved sizing measured, using the Hercules Size Test solution No. 2 to 80% reflectance. This measures the surface sizing of the paper. However, there is no suggestion that there is a particular problem caused by sterilization by hydrogen peroxide, nor is there any indication that the sizing using the combined cationic dispersion has any effect on resistance to edge penetration by hydrogen peroxide containing liquids nor by liquids containing lactic acid following contact with hot hydrogen peroxide solutions.
There is no known correlation between surface sizing and edge penetration resistance--good surface sizing does not ensure good edge penetration resistance. Further the measurement of effects of aggressive liquids such as hot hydrogen peroxide containing liquids and lactic acid can give very different results depending on the tests used for such measurements.
In fact, as shown by the introduction to the said U.S. patent, the object of the invention is not only to improve sizing per se but to overcome several of the well known disadvantages associated with either the cellulose reactive sizing agent or the cationic rosin size alone, and particularly to improve the rate at which sizing develops. If sizing develops quickly, newly formed paper is able to run more quickly through the machine, and in particular through the sizing press, which is clearly advantageous from an operational viewpoint for the production of paper, but has no relevance to the production of paper board in the absence of a sizing press, and its properties in use.